Hydrogen adsorption in a highly stable porous rare-earth metal-organic framework: sorption properties and neutron diffraction studies

A highly stable porous lanthanide metal-organic framework, Y(BTC)(H2O).4.3H2O (BTC = 1,3,5-benzenetricarboxylate), with pore size of 5.8 A has been constructed and investigated for hydrogen storage. Gas sorption measurements show that this porous MOF exhibits highly selective sorption behaviors of hydrogen over nitrogen gas molecules and can take up hydrogen of about 2.1 wt % at 77 K and 10 bar. Difference Fourier analysis of neutron powder diffraction data revealed four distinct D2 sites that are progressively filled within the nanoporous framework. Interestingly, the strongest adsorption sites identified are associated with the aromatic organic linkers rather than the open metal sites, as occurred in previously reported MOFs. Our results provide for the first time direct structural evidence demonstrating that optimal pore size (around 6 A, twice the kinetic diameter of hydrogen) strengthens the interactions between H2 molecules and pore walls and increases the heat of adsorption, which thus allows for enhancing hydrogen adsorption from the interaction between hydrogen molecules with the pore walls rather than with the normally stronger adsorption sites (the open metal sites) within the framework. At high concentration H2 loadings (5.5 H2 molecules (3.7 wt %) per Y(BTC) formula), H2 molecules form highly symmetric novel nanoclusters with relatively short H2-H2 distances compared to solid H2. These observations are important and hold the key to optimizing this new class of rare metal-organic framework (RMOF) materials for practical hydrogen storage applications.     

Rational construction of 3D pillared metal-organic frameworks: synthesis, structures, and hydrogen adsorption properties

In our efforts toward rational design and systematic synthesis of 'pillar-layer' structure MOFs, three porous MOFs have been constructed based on [Zn(4)(bpta)(2)(H(2)O)(2)] (H(4)bpta = 1,1'-biphenyl-2,2',6,6'-tetracarboxylic acid) layers and three different bipyridine pillar ligands. The resulted MOFs show similar structures but different pore volume and window size depending on the length of pillar ligands which resulted in distinct gas adsorption properties. In the three MOFs, [Zn(4)(bpta)(2)(4,4'-bipy)(2)(H(2)O)(2)]·(DMF)(3)·H(2)O (1) (DMF = N,N'-dimethylformamide and 4,4'-bipy = 4,4'-bipyridine) reveals selective adsorption of H(2) over N(2) and O(2) as the result of narrow pore size. [Zn(4)(bpta)(2)(azpy)(2)(H(2)O)(2)]·(DMF)(4)·(H(2)O)(3) (2) and [Zn(4)(bpta)(2)(dipytz)(2)(H(2)O)(2)]·(DMF)(4)·H(2)O (3) (azpy =4,4'-azopyridine, dipytz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) reveal pore structure change upon different activation conditions. In addition, the samples activated under different conditions show distinct adsorption behaviors of N(2) and O(2) gases. Furthermore, hydrogen adsorption properties of activated 1-3 were studied. The results indicated that the activation process could affect the hydrogen enthalpy of adsorption.     

Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.     

Understanding adsorption and interactions of alkane isomer mixtures in isoreticular metal-organic frameworks

Novel metal-organic frameworks (MOFs) may lead to advances in adsorption and catalysis owing to their superior properties compared to traditional nanoporous materials. A combination of the grand canonical Monte Carlo method and configurational-bias Monte Carlo simulation was used to evaluate the adsorption isotherms of C4-C6 alkane isomer mixtures in IRMOF-1 and IRMOF-6. The amounts of adsorbed linear and branched alkanes increase with increasing pressure, and the amount of branched alkanes is larger than that of the linear ones. The locations of the alkane isomer reveal that the Zn4O clusters of the IRMOFs are the preferential adsorption sites for the adsorbate molecules. The interaction energy between the Zn4O cluster and the adsorbate is larger than that between the organic linker and the adsorbate. It was further confirmed that the Zn4O cluster plays a much more important role in adsorption by pushing a probe molecule into the pore at positions closer to the Zn4O cluster. It is difficult for branched alkane molecules to approach the Zn4O cluster of IRMOF-6 closely owing to strong spatial hindrance. In addition, the adsorption selectivity is discussed from the viewpoints of thermodynamics and kinetics, and the diffusion behavior of n-butane and 2-methylpropane were investigated to illustrate the relationship between diffusion and adsorption.     

金催化一氧化碳氧化反应中锌锡复合氧化物的载体效应

氧化物负载的纳米金催化剂对CO氧化反应具有极高的活性,这不仅依赖于金的结构特性,也取决于氧化物载体的结构.近年来,除了氧化硅、氧化铝等惰性载体以及氧化钛、氧化铈、氧化铁等可还原性载体外,人们还致力于探索各类新型氧化物载体.另一方面,锡酸锌是具有反尖晶石结构的化合物,并且在透明导电氧化物、锂离子电池阳极材料、光电转换装置以及传感器等方面应用广泛.然而,迄今为止,锡酸锌仍未被用于负载纳米金催化剂,因此相关的构效关系作用研究也十分有限.基于此,本文采用氮气吸附-脱附实验、电感耦合等离子体原子发射光谱(ICP-AES)、X射线衍射(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)和高分辨电镜(HRTEM)、高角环形暗场像-扫描透射电子显微镜(HAADF-STEM)、X射线吸收精细结构谱(XAFS)和氢气程序升温脱附(H2-TPD)等手段,系统研究了锡酸锌负载的纳米金催化剂在CO氧化反应中催化性能差异的原因.首先,利用水热法制备了锡酸锌(ZTO)载体,而其织构性质可由碱(N2H4·H2O)与金属离子(Zn2+)的比例在4/1(ZTO_1)、8/1(ZTO_2)和16/1(ZTO_3)之间进行调节.结果发现, ZTO_2具有最大的孔体积(0.223 cm3/g)和最窄的孔径分布.再采用沉积沉淀法将0.7 wt% Au负载于其上,得到金-锡酸锌(Au_ZTO)催化剂. ICP-AES测得样品中Au含量在0.57-0.59 wt%,与投料比接近. CO氧化反应结果显示, Au_ZTO_1和Au_ZTO_2的表观活化能相同,但后者的活性更高;而Au_ZTO_3在220°C以下没有活性,催化性能最差,与纯锡酸锌载体相当. XRD结果显示,反应过程中ZTO晶相、晶胞参数及晶粒尺寸变化不明显; TEM和HRTEM分析表明,载体ZTO在反应前后均为多面体形貌,平均颗粒尺寸在12-16 nm; XPS结果验证了Zn2+和Sn4+离子是新鲜和反应后样品中载体金属的存在形式; HAADF-STEM探测到所有样品中均含有1-2 nm的Au粒子; XAFS结果表明, Au以Au0形式存在,并且在Au_ZTO_3中Au平均粒径大于4 nm,而其它两样品约为2 nm. H2-TPR结果表明,金的引入对ZTO载体耗氢量影响不大,但还原峰温度向低温移动;金属-载体相互作用强弱与催化活性高低具有正相关性,即Au_ZTO_2> Au_ZTO_1>> Au_ZTO_3.这是由于不同织构性质的锡酸锌载体对于纳米金活性物种的稳定作用不同所致,具有最大孔体积和最窄孔径分布的ZTO_2负载的金纳米颗粒表现出最高活性.     

A metal-organic framework with highly polar pore surfaces: selective CO2 adsorption and guest-dependent on/off emission properties

A 3D porous Zn(II) metal-organic framework {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)(H(2)O)]·4H(2)O} (1; H(2)dht=dihydroxyterphthalate, azpy=4,4'-azobipyridine) has been synthesised by employing 2,5-dihydroxyterephthalic acid (H(4)dht), a multidentate ligand and 4,4'-azobipyridine by solvent-diffusion techniques at room temperature. The as-synthesised framework furnishes two different types of channels: one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The dehydrated framework, {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)]} (1') provides 52.7% void volume to the total unit-cell volume. The pore surfaces of 1' are decorated with unsaturated Zn(II) sites and pendant hydroxyl groups of H(2)dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1' shows highly selective adsorption of CO(2) over other gases, such as N(2), H(2), O(2) and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission (λ(max)≈530 nm) on the basis of the excited-state intramolecular proton-transfer (ESIPT) process of the H(2)dht linker; no emission was observed in dehydrated solid 1'. Such guest-induced on/off emission has been correlated to the structural transformation and concomitant breaking and reforming of the OH···OCO hydrogen-bonding interaction in the H(2)dht linker in 1'/1.     

Zn-based metal organic framework derivative with uniform metal sites and hierarchical pores for efficient adsorption of formaldehyde

A Zn-containing graphite carbon (Zn-GC) with uniform Zn metal sites and hierarchical pore structure was obtained by pyrolysis of Zn-based metal organic framework (MOF). Zn-GC exhibited excellent adsorption capacity and reproducibility for formaldehyde. The adsorption capacity of Zn-GC was 736 times that of commercial activated carbon and 5.6 times that of ZSM-5 adsorbents. The characterization and experimental results showed that the surface chemical characteristics of the adsorption material play an important role in the adsorption performance. The superior performance was attributed to Zn metal sites and oxygen-containing functional groups on the MOF derivative as well as hierarchical pore structure. The material showed a great potential in the field of organic pollutant removal.     

Periodic DFT calculations of the stability of Al/Si substitutions and extraframework Zn2+ cations in mordenite and reaction pathway for the dissociation of H2 and CH4

The local stability of Al atoms replacing Si in the zeolite framework is compared for all inequivalent tetrahedral (T) sites in mordenite. For Al/Si substitutions in two T sites the stable location of the compensating extraframework Zn(2+) cation forming a Lewis acid site is determined. In the most stable Zn-MOR structures Zn(2+) is located in a small ring (5MR, 6MR) containing two Al/Si substitutions. In less stable structures the Al atoms are placed at larger distances from each other and Zn(2+) interacts with only one Al site. The simulated adsorption of H(2) and CH(4) shows that adsorption strength decreases with increasing stability of the Zn(2+) Lewis site. A higher adsorption strength is observed for Zn(2+) deposited in the 5MR than for the 6MR. The reactivity of a series of stable Zn(2+) Lewis sites is tested via the dissociative adsorption of H(2) and CH(4). The heterolytic dissociation of the adsorbed molecule on the extraframework Zn(2+) cation produces a proton and an anion. The anion binds to Zn(2+) and proton goes to the zeolite framework, restoring a Br?nsted acid site. Because bonding of the anion to Zn(2+) is almost energetically equivalent for Zn(2+) in any of the extraframework positions the dissociation is governed by stabilizing bonding of the proton to the framework. Those structures which can exothermically accommodate the proton represent reaction pathways. Due to the repulsion between the proton and Zn(2+) the most favorable proton-accepting O sites are not those of the ring where Zn(2+) is deposited, but O sites close to the ring. Large differences are observed for neighboring positions in a- and b-directions and those oriented along the c-vector. Finally, among the stable Zn(2+) Lewis sites not all represent reaction pathways for dehydrogenation. For all of them the dissociation of H(2) is an exothermic process. In structures exhibiting the highest reactivity the Al/Si substitutions are placed at a large distance and the Zn(2+) cation interacts with O-atoms next to Al in the T4 site of the 5MR. This Lewis site is strong enough to break the C-H bond in the CH(4) molecule.     

Design for hydrogen storage materials via observation of adsorption sites by computer tomography

An effective method denoted as "computer tomography for materials" (mCT) was employed to study the adsorption sites inside metal-organic frameworks (MOFs) at any positions and any view angles. For MOF-5, the first adsorption site alpha(-COO)3 was clearly observed from the mCT images; it locates at the position where three -COO groups joined like a cup. There are four alpha(-COO)3 sites around the Zn4O cluster. Two of them located at the diagonal of the Zn4O cluster are in the same plane "A", whereas the other two equivalent adsorption sites are in another plane "B", which is about 5.4 A away from the plane A. It was found that the electronegativity of oxygen atoms is very important to the adsorption of hydrogen molecules. The hydrogen amount adsorbed in MOFs might be enhanced by introducing some strong electronegative atoms to the organic linkers or frameworks. On the basis of this point of view, five new MOF materials were designed. The adsorbed amounts both in number of hydrogen molecules per unit cell and weight uptake for all of the designed MOFs were calculated. The adsorption amounts of designed MOFs were improved, and the amount for MOF-d5 at 1 bar is as high as 3.7 wt %. It is nearly 5-6 times of that of MOF-5 as a whole. It can be observed that extra adsorption sites were formed in the pores and the effective occupation rate of pore space was obviously improved viewing from the mCT images. These results may give helpful suggestions for the synthetic experimentalists.     

Surface dependence of CO2 adsorption on Zn2GeO4

An understanding of the interaction between Zn(2)GeO(4) and the CO(2) molecule is vital for developing its role in the photocatalytic reduction of CO(2). In this study, we present the structure and energetics of CO(2) adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn(2)GeO(4) (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn(2)GeO(4) is important for CO(2) adsorption and activation, i.e., the interaction of CO(2) with Zn(2)GeO(4) surfaces is structure-dependent. The ability of CO(2) adsorption on (001) is higher than that of CO(2) adsorption on (010). For the (010) surface, the active sites O(2c)···Ge(3c) and Ge(3c)-O(3c) interact with the CO(2) molecule leading to a bidentate carbonate species. The presence of Ge(3c)-O(2c)···Ge(3c) bonds on the (001) surface strengthens the interaction of CO(2) with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO(2) adsorption on perfect and defective Zn(2)GeO(4) (010) and (001) surfaces shows that CO(2) has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy -1.05 to -2.17 eV, stronger than adsorption of CO(2) on perfect Zn(2)GeO(4) surfaces (E(ads) = -0.91 to -1.12 eV) or adsorption of CO(2) on a surface oxygen defect site (E(ads) = -0.24 to -0.95 eV). Additionally, for the defective Zn(2)GeO(4) surfaces, the oxygen vacancies are the active sites. CO(2) that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO(2) on the surface oxygen defect site, we concluded that dissociative adsorption of CO(2) favors the stepwise dissociation mechanism and the dissociation process can be described as CO(2) + Vo → CO(2)(δ-)/Vo → CO(adsorbed) + O(surface). This result has an important implication for understanding the photoreduction of CO(2) by using Zn(2)GeO(4) nanoribbons.     

Interaction of Hydrogen with MOF-5

Hydrogen storage is among the most demanding challenges in the hydrogen-based energy cycle. One proposed strategy for hydrogen storage is based on physisorption on high surface area solids such as metal-organic frameworks (MOFs). Within this class of materials, MOF-5 has been the first structure studied for hydrogen storage. The IR spectroscopy of adsorbed H2 performed at 15 K and ab initio calculations show that the adsorptive properties of this material are mainly due to dispersive interactions with the internal wall structure and to weak electrostatic forces associated with O13Zn4 clusters. Calculated and measured binding enthalpies are between 2.26 and 3.5 kJ/mol, in agreement with the H2 rotational barriers reported in the literature. A minority of binding sites with higher adsorption enthalpy (7.4 kJ/mol) is also observed. These species are probably associated with OH groups on the external surfaces present as termini of the microcrystals.     

Raman studies of hydrogen adsorbed on nanostructured porous materials

Raman spectroscopy was applied to study the adsorbed hydrogen phase in porous materials at room temperature and under cryogenic conditions. A comparison between the Raman spectra of H(2) molecules adsorbed on single walled carbon nanotubes and on a Cu-based metal-organic framework reveals that the interaction strength for the adsorption of molecular hydrogen is very similar in these materials. In both cases the small perturbation of the Raman spectrum of hydrogen indicates that adsorption takes place without any evident charge transfer between H(2) and the adsorbent. Additionally for single walled carbon nanotubes at least two types of adsorption sites could be identified by Raman spectroscopy.     

A molecular shuttle for driving a multilevel fluorescence switch

A [2]rotaxane-based molecular shuttle comprised a macrocycle mechanically interlocked to a chemical "dumbbell" has been prepared in high yields by a thermodynamically controlled, template-induced clipping procedure. This molecular shuttle has two different recognition sites, namely, -NH2 +- and amide, separated by a phenyl unit. The macrocycle exhibits high selectivity for the -NH2+- recognition sites in the protonated form through noncovalent interactions, which include 1) N+-H...O hydrogen bonds; 2) C-H...O interactions between the CH2NH2+CH2 protons on the thread and the oligo(ethylene glycol) unit in the macrocycle; 3) pi...pi stacking interaction between macrocycle and aromatic unit. Upon deprotonation of the [2]rotaxane the macrocycle glides to the amide recognition site due to the hydrogen bonds between the -CONH- group and the oligo(ethylene glycol) unit in the macrocycle. The deprotonation process requires about 10 equivalents of base (iPr2NEt) in polar acetone, while the amount of base is only 1.2 equivalents in apolar tetrachloroethane. Upon addition of Li+, the conformation of the [2]rotaxane was altered as a result of the collective interactions of 1) hydrogen bonds between pyridine nitrogen and amide hydrogen atoms; 2) coordination between the oligo(ethylene glycol) unit, amide oxygen atom and Li+ cation. Then, when Zn2+ ions are added, the macrocycle returns to the deprotonated -NH- recognition site owing to coordination of the macrocycle and -NH- from the axle with the Zn2+ ion. All the above-mentioned movement processes are reversible through the alternate addition of TFA/iPr2NEt, Li/[12]-crown-4 and Zn2+/ethylenediaminetetraacetate (EDTA), by virtue of hydrogen bonding and metal-ion complexation. Significantly, the three independent movement processes are all accompanied by fluorescent responses: 1) complete repression in the protonated form; 2) low-level expression in the deprotonated form; 3) medium-level expression following addition of Li+; 4) high-level expression on complexation with Zn2+.     

Hydrogen thermal desorption spectra: insights from molecular simulation

Thermal desorption spectra of a number of metal-organic frameworks were studied using grand canonical Monte Carlo simulation. Our simulation results are in qualitative agreement with experimental results but also show that great care must be taken when choosing the force field to describe the hydrogen/framework interaction. As the simulations additionally yield the positions and potential energies of the adsorbed molecules it is straightforward to assign the peaks and features in the thermal desorption spectra to specific adsorption sites. We show that the location of the peaks is directly related to the hydrogen-framework interaction which is a complex function of the chemical and topological environment of the pore space, the pore size and the presence of specific interaction sites such as open metal sites. Finally, we demonstrate that an IRMOF-8 sample used to obtain an experimental thermal desorption spectrum must have indeed been catenated as previously suspected. Overall, molecular simulation is a useful tool to complement the interpretation of experimental thermal desorption spectra.     

A porous framework polymer based on a zinc(II) 4,4'-bipyridine-2,6,2',6'-tetracarboxylate: synthesis, structure, and "zeolite-like" behaviors

The robust metal-organic framework compound {[Zn(2)(L)] x 4H(2)O}(infinity) I has been synthesized by hydrothermal reaction of ZnCl(2) and 4,4'-bipyridine-2,6,2',6'-tetracarboxylic acid (H(4)L). Compound I crystallizes in a chiral space group, P4(2)2(1)2, with the chirality generated by the helical chains of hydrogen-bonded guest water molecules rather than by the coordination framework. Removal of guest water molecules from the crystal affords the porous material, [Zn(2)(L)](infinity) (II), which has very high thermal stability and is chemically inert. The N(2) isotherm of II at 77 K suggests a uniform porous structure with a BET surface area of 312.7 m(2)/g and a remarkably strong interaction with N(2) molecules (betaE(0) = 29.6 kJ mol(-)(1)). II also exhibits significant gas storage capacities of 1.08 wt % for H(2) at 4 bar and 77 K and 3.14 wt % (44.0 cm(3)/g, 67 v/v) for methane at 9 Bar at 298 K. The adsorption behavior of II toward organic solvent vapors has also been studied, and isotherms reveal that for different solvent vapors adsorption is dominated by two types of processes, absorbate-absorbate or absorbate-absorbent interactions. The adsorption and desorption kinetic processes in II are determined mainly by the molecular size of the guest species and their interaction with the host.     

Capture of nerve agents and mustard gas analogues by hydrophobic robust MOF-5 type metal-organic frameworks

In this communication, a series of observations and data analyses coherently confirms the suitability of the novel metal-organic framework (MOF) [Zn(4)(μ(4)-O)(μ(4)-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)(3)] (1) in the capture of harmful volatile organic compounds (VOCs). It is worthy of attention that 1, whose crystal structure resembles that of MOF-5, exhibits remarkable thermal, mechanical, and chemical stability, as required if practical applications are sought. In addition, it selectively captures harmful VOCs (including models of Sarin and mustard gas, which are chemical warfare agents), even in competition with ambient moisture (i.e., under conditions mimicking operative ones). The results can be rationalized on the basis of Henry constant and adsorption heat values for the different essayed adsorbates as well as H(2)O/VOC partition coefficients as obtained from variable-temperature reverse gas chromatography experiments. To further strengthen the importance of 1, its performance in the capture of harmful VOCs has been compared with those of well-known materials, namely, a MOF with coordinatively unsaturated metal sites, [Cu(3)(btc)(2)] and the molecular sieve active carbon Carboxen. The results of this comparison show that coordinatively unsaturated metal sites (preferential guest-binding sites) are ineffective for the capture of VOCs in the presence of ambient moisture. Consequently, we propose that the driving force of the VOC-MOF recognition process is mainly dictated by pore size and surface hydrophobicity.     

Hypothetical high-surface-area carbons with exceptional hydrogen storage capacities: open carbon frameworks

A class of high-surface-area carbon hypothetical structures has been investigated that goes beyond the traditional model of parallel graphene sheets hosting layers of physisorbed hydrogen in slit-shaped pores of variable width. The investigation focuses on structures with locally planar units (unbounded or bounded fragments of graphene sheets), and variable ratios of in-plane to edge atoms. Adsorption of molecular hydrogen on these structures was studied by performing grand canonical Monte Carlo simulations with appropriately chosen adsorbent-adsorbate interaction potentials. The interaction models were tested by comparing simulated adsorption isotherms with experimental isotherms on a high-performance activated carbon with well-defined pore structure (approximately bimodal pore-size distribution), and remarkable agreement between computed and experimental isotherms was obtained, both for gravimetric excess adsorption and for gravimetric storage capacity. From this analysis and the simulations performed on the new structures, a rich spectrum of relationships between structural characteristics of carbons and ensuing hydrogen adsorption (structure-function relationships) emerges: (i) Storage capacities higher than in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets, which creates surface areas exceeding of 2600 m(2)/g, the maximum surface area for infinite graphene sheets, carried mainly by edge sites; we call the resulting structures open carbon frameworks (OCF). (ii) For OCFs with a ratio of in-plane to edge sites ≈1 and surface areas 3800-6500 m(2)/g, we found record maximum excess adsorption of 75-85 g of H(2)/kg of C at 77 K and record storage capacity of 100-260 g of H(2)/kg of C at 77 K and 100 bar. (iii) The adsorption in structures having large specific surface area built from small polycyclic aromatic hydrocarbons cannot be further increased because their energy of adsorption is low. (iv) Additional increase of hydrogen uptake could potentially be achieved by chemical substitution and/or intercalation of OCF structures, in order to increase the energy of adsorption. We conclude that OCF structures, if synthesized, will give hydrogen uptake at the level required for mobile applications. The conclusions define the physical limits of hydrogen adsorption in carbon-based porous structures.     

Synthesis of isoreticular zinc(II)-phosphonocarboxylate frameworks and their application in the Friedel-Crafts benzylation reaction

Three isoreticular zinc(II)-phosphonocarboxylate frameworks, namely {[Zn(3)(pbdc)(2)]·2H(3)O}(n) (ZnPC-2), {[Zn(3)(pbdc)(2)]·Hpd·H(3)O·4H(2)O}(n) (Hpd@ZnPC-2) and {[Co(1.5)Zn(1.5)(pbdc)(2)]·2H(3)O}(n) (CoZnPC-2) (H(4)pbdc=5-phosphonobenzene-1,3-dicarboxylic acid, pd=pyrrolidine), were solvothermally synthesized. ZnPC-2 has a 3D structure based on trinuclear Zn(II) clusters (Zn(3)-SBU) showing 3D interconnected channels. Hpd@ZnPC-2 contains an isoreticular framework of ZnPC-2 with small channels blocked by Hpd molecules. In CoZnPC-2, Zn(II) ions in ZnPC-2 are partially substituted by Co(II) ions. The Friedel-Crafts benzylation reactions were carried out over these isoreticular porous materials. The catalytic results reveal that ZnPC-2 is an excellent heterogeneous Lewis acid catalyst with a high selectivity (>90%) towards less bulky para-oriented products. The catalytic reaction has been proved to occur inside the pore of ZnPC-2, and the immobilized Zn(3)-SBUs are the active sites.     

Multifunctionality and crystal dynamics of a highly stable, porous metal-organic framework [Zn4O(NTB)2

A porous metal-organic framework [Zn(4)O(NTB)(2)].3DEF.EtOH (1), in which (3,6)-connected nets are doubly interpenetrated to generate curved three-dimensional channels, has been prepared. Framework 1 exhibits high permanent porosity (Langmuir surface area, 1121 m(2)/g; pore volume, 0.51 cm(3)/cm(3)), high thermal stability (up to 430 degrees C), high hydrogen adsorption capacity (1.9 wt % at 77 K and 1 atm), selective organic guest binding ability (K(f)()( )(): MeOH > pyridine > benzene > dodecane), and guest-dependent blue luminescence (lambda(max) depending on guest identity). Most interestingly, the framework sustains single crystallinity even at 400 degrees C and 10(-)(5) Torr, and the framework components undergo reversible dynamics, mainly rotational motion, in response to removal and rebinding of the guest molecules.     

Functionalities of one-dimensional dynamic ultramicropores in nickel(II) coordination polymers

Ni(II) coordination polymers with a 4,4'-azobis(pyridine) (azpy) ligand, {[Ni2(NCX)4(azpy)4].G}n (X = S, G (guest molecule) = MeOH (1.MeOH); X = S, G = EtOH (1.EtOH); X = S, G = H2O (1.H2O); X = S, G = no guest (1); X = Se, G = MeOH (2.MeOH); X = Se, G = H2O (2.H2O); X = Se, G = no guest (2)), have been synthesized and structurally characterized with their porosity. These compounds have one-dimensional periodic ultramicropores that contain the small guest molecules, H2O, MeOH, or EtOH, whose hydroxy groups interact with the S or Se atoms of isothiocyanate or isoselenocyanate, respectively, via -S(Se)...HO- hydrogen bonds. Although the molecular dimensions of the MeOH guest are considerably larger than the window size of the ultramicropore, 1.MeOH and 2.MeOH easily release their guest molecules without decomposition of the framework to form 1 and 2 without any guest molecules. This shows that 1 and 2 have dynamic ultramicropores constructed from the interpenetrating framework. The guest desorption experiments using 1.MeOH and 1.EtOH reveal that the difference in the desorption behavior is due to van der Waals interactions that depend on the molecular shape of the guest molecule in the ultramicropores and/or an entrance blocking effect that depends on the minimum dimensions of the guest molecule for the pore windows. A marked difference in the N2 and CH4 adsorption isotherms was observed and is associated with the strength of the host-guest interaction.